Sound reproduction



April 26, 1938. R, BALSLEY 2,115,001

SOUND REPRODUCTION Filed Oct. 29, 1935 2 Sheets-Sheet 1 INVENTOR. N \24JAMES P. BALsLEY By W ATTORN April 6, 1938. J. R. BALSLEY 2,115,001

SOUND REPRODUCTION Filed Oct. 29, 1935 2 Sheets-Sheet 2 PIE. 3.

JNVENTOR. JAMES R BALSLEY ATTORNEY Patented Apr. 26, 1938 UNITED STATESPATENT OFFICE SOUND REPRODUCTION James R. Balsley, La Canada,

Califi, assignor to Application October 29, 1935, Serial No. 47,288

13 Claims.

This invention relates to sound reproduction and particularly to thereproduction of sound from a record in which during reproduction thevolume level is automatically controlled.

In most of the systems of the prior art, a portion of the sound currentsare rectified, filtered and are impressed either upon a vacuum tube tochange the amplification factor in accordance with the amplitude of thesound modulations, or are impressed directly upon a transmission elementfor the sound modulated currents so as to vary the change in the volumelevel range. Filter circuits necessary to filter or smooth out therectified audio current having an alternating cur rent component thereonintroduce time lag and other deleterious effects which may distort thesound reproduced.

One object of the present invention is to eliminate the use of filtersfor smoothing out the rectified audio frequency current employed tochange the volume level range of an automatic volume controllingcircuit. This is accomplished by feeding the rectified current obtainedthrough the photoelectric cell due to variations in the average printtransmission through the film and/or the rectified sound currents into atransmission element for transmitting the audio fre quency soundcurrents into a sound reproducer. This rectified current is divided intotwo paths connecting the detector or rectifying device to a suitablereceiving circuit such as a bridge, in the audio current transmissionelement. A phase changing device is provided in one of the paths tochange the phase of the alternating current components in thatparticular path 180. Also in one of the paths the current variations dueto both the average print intensity and rectification appear. Thus whenthe divided rectified current is fed into the transmission element thealternating current components will be neutralized or balanced out dueto the phase opposition of the two current conducting paths leaving onlythe smooth varying direct current to be fed into the transmissionelement to change the volume levels of the reproduced sound. The amountof direct current thus obtained may be in accordance with the variationin average current fiow through the cell due to the average printtransmission of the sound track portion of the film which in the case ofan anti-ground noise type of film varies practically directly as thevolume of the sound, or it may vary in accordance with the rectifiedsound current, or it may vary in accordance with both.

Other objects and advantages of this circuit for expanding sound will bemore fully appreciated and the invention more fully understood byreference to the following description read in conjunction with theaccompanying drawings in which:

Fig. 1 is a schematic circuit diagram of a sound reproducing apparatusembodying the invention.

Fig. 2 is an enlarged section of a film having a variable area soundtrack of the anti-ground noise type; and

Fig. 3 is a schematic circuit diagram of a modified form of theinvention.

Referring to Fig. 1 the sound record film i may be either of thevariable area or variable density type. Also it may have an averageprint transmission which is proportional to the amplitude of the soundrecorded, as in the so-called antiground noise type of record, or it maybe of the normal unbiased and unshuttered type of record in which theaverage print transmission is sub stantially the same throughout thelength of the record. In speaking of average transmission I mean theratio of average transparency to average opacity of a section of soundtrack which covers more than one sound wave length representation.

Light from an exciter lamp 2 projects a beam of light through acondenser lens 3 and onto the film l in the form of a fine horizontalline of light. The beam of light thus modulated in frequency andintensity by the film l is focused by objective lens 4 upon aphotoelectric cell 5. In series with the photoelectric cell 5 areresistance 6, a polarizing battery 1, and a load resistance 8. One sideof the load resistance 8 is grounded at 9 and the other side isconnected through lead ID with the grid of a detector tube l I. Theplate circuit of tube II comprises the primary of a transformer l2,plate battery 13 and potentiometer resistance H in series. The filamentof tube II is grounded at l5 to complete the grid circuit while thesecondary of transformer i2 is placed in series with a potentiometerresistance i4. Contacts 43 and 44 along the resistance l4 and I1 areconnected through leads IB and I 9 respectively to the grids of tubesand 2| respectively. Biasing batteries 22 and 23 are provided in thegrid circuits to keep a negative potential upon the grids of tubes 20and 2|. and obtain a static balance. These tubes 20 and 2| constitutetwo arms of a bridge circuit 24. Also included in either arm of thebridge circuit 24 is a pair of balanced secondaries 25 and 26 of atransformer 21. A plate battery 28 and output resistance 29 areconnected in series across the junction of the two secondary windings 25and 26 and the junction of the filaments of the two tube-s 20 and 2!.

The primary of transformer 27 constituting part of the audio currenttransmission circuit comprises, with a plate battery 30 in seriestherewith, the plate circuit of an amplifier tube 3|. The grid of tube3| is connected to the photoelectric cell circuit through a lead 32having a condenser 33 in series therewith. A grid biasing battery 34 andresistance 35 is provided between the grid and filament of tube 3! tomaintain a negative potential on the grid of tube 3|. The filament oftube 3| is grounded to complete the grid circuit across thephotoelectric cell 5.

A lead 36 and ground connection 31 are connected across the resistance29 and battery 28 of the bridge circuit 24 thus forming a grid circuitfor an amplifying tube 38. A biasing battery 39 and resistance 40 areconnected across the grid and filament of tube 38 to maintain a.negative potential upon the grid of tube 38. The output of the platecircuit of tube 38 passes through a transformer 4|. through furtheramplifying equipment (not shown) if desired, and thence into the loudspeaker 42.

In the operation of the apparatus the battery 1 produces a constantpotential upon. the photoelectric cell 5. The varying intensity of thelight beam reaching the photoelectric cell 5 causes a drop in potentialacross the resistance 8 which varies in accordance with the printtransmission of the film and, without condenser 46 shown in dottedlines, in accordance with the sound currents. This drop in potential isapplied through the lead it! upon the grid of the detector tube H. Thusthere is applied to the grid of tube Ii an alternating current voltageproduced by the sound modulations as well as a direct current voltagedue to the average photoelectric cell current produced by the averageprint transmission. The alternating components of output currents of theplate circuit of tube II when transferred to the secondary oftransformer I2 is displaced 180 in phase relation so that the two leadsl8 and I9 carry equal and opposite impulses of alternating current. Thesliding contacts 43 and 44 are so adjusted along the potentiometerresistances l4 and H that during the passage of a normal unmodulatedportion of the film before the photoelectric cell 5 the bridge circuit24 will be in balance. Upon entering the bridge circuit 24 thesealternating current component impulses are neutralized due to their 180phase relation leaving only the direct current variations applied fromcontact 44 to change the impedance of the tube 2| and thus unbalance thebridge circuit 24 to produce a voltage drop across the resistance 29which varies in accordance with the average print transmission of thefilm and in accordance with the volume variation of the sound by thecurrents produced by rectification when condenser 46 is not employed.This voltage is then applied through lead 35 and ground connection 31 tothe amplifier tube 38.

The frequency of the sound reproduced is determined by the currentpassing through the lead 32, condenser 33, and tube 3|. The output oftube 3!, having a frequency equal to that of the recorded sound, isapplied across the transformer 21 and into the plate circuits of thetubes 20 and 2|.

When condenser 46 is in circuit and by-passes the alternating componentsfrom cell 5, or the sound modulations, then unbalance of bridge 24occurs only by currents produced by the average print transmission ofthe sound track.

It is thus realized that in using a film of the anti-ground noise typesuch as shown in Fig. 2, where the average print transmission varies inaccordance with the amplitude of the sound modulations, both currentsproduced by this variation plus those produced by rectification may beused to disturb the balance of bridge 24 to obtain volume expansion inaccordance therewith. To utilize both currents, condenser 46 is notused, this condenser being solely for the purpose of eliminating thealternating current modulations from the grid of tube II and volumeexpansion is to be in accordance with average print transmission only.In the case, however, of a record without noise reduction, thencondenser 46 is not employed in order to obtain on the grid of tube I lthe sound modulation currents which through rectification in tube IIproduced the unbalancing desired.

Attention is directed to the method of and circuit employed foreliminating filters in the output of the rectifier which consists of notonly reversing the phase of the alternating current components andimpressing both phase opposition currents upon the grids of the tubes inthe bridge circuit, but also eliminates the direct current componentsfrom one of the grid circuits, thus obtaining an unbalance of thebridge.

Referring now to Fig. 3 the same elements 1-5 inclusive are employed asshown in Fig. 1 while the output of this system may comprise the usualamplifier 5D and loud speaker 5|. This circuit also employs a bridge 54including two tubes 55 and 56 but differs from the bridge circuit 24 ofFig. 1 by employing an output transformer 58 and a tapped balancingresistance 59 for aiding the adjustment of balance of the plate circuitsof tubes 55 and 56. The alternating current components are impressed onthe bridge through the condensers Gil and amplifier tube 5! having aplate circuit including a primary of the transformer 63 and a platebattery 64 and having a grid circuit including a resistance 65 andbiasing battery 66, to maintain a negative potential on the grid of tube6|. The photoelectric cell 5 is polarized over a load resistance 69 anda resistance 10 by a source of potential "H which also supplies platepotential to a detector tube 13. Tube 13 has a grid circuit employing aresistance 10 and potentiometer resistance 15, which supplies grid biasto the tube 13 and also is a load resistance of the tube 13 forproducing a potential for impression upon grids of tubes 55 and 55. Amid-tap 16 is connected to the cathodes of tubes 55 and 56 While theterminals of resistance 15 are connected to the grids of the tubes 55and 56. Voltage created across the upper half of resistance '15 istherefore impressed on the tube of grid 56 and the voltage produced onthe lower half is impressed upon the grid of tube 55. Since the lowerterminal resistance 15 will be negative when the upper terminal ispositive, alternating components are impressed on the grids of tubes 55and 56 in phase opposition and will thus be balanced out in the platecircuit of the bridge tubes or, in other words, in the transformer 58,thus producing no output current. The direct current component, however,flowing in resistance 15 changes the biasing potential thereon at a rateto unbalance both sides of the bridge composed of the cathode anodecircuits of the respective tubes, thus permitting the alternatingcurrent components of cell 5 amplified by tube 6| to be transmitted toamplifier 50 at an amplitude dependent upon the unbalance of bridge 54.This unbalance is directly proportional to the direct current componentin the plate circuits of tube 13 flowing through resistance 15.

Similar to the circuit of Fig. 1 a condenser 18 may or may not beemployed to shunt resistance 10, that is, if it is desired to obtainunbalance of bridge 54 solely by the variations in the printtransmission of an anti-ground noise record shown in Fig. 2, the soundmodulations may be by-passed across resistance 16, the direct currentcomponents in resistance 15 being due therefore only to the averageprint transmission. However, with the condenser out of circuit, then thedirect currents occurring in resistance 15 are due to both therectification of the sound modulations and the variations in printtransmission in the case of an anti-ground noise record and only to therectified sound currents in the case of the record not having the noisereduction feature.

What is claimed is:

l. A system of sound reproduction comprising a source of audio frequencycurrents, means for obtaining from said currents a current varyingsubstantially in accordance with the amplitude variations thereof, meansfor obtaining said audio frequency currents in phase opposition, abalanced transmission element for transmitting said audio frequencycurrents, and means for impressing said amplitude varying currents onone portion of said transmission element and said phase oppositioncurrents on two similar portions of said transmission element includingsaid first portion.

2. In a sound reproducing system, the combination of a pair of vacuumtubes, each having a cathode, a plate and a grid and having platecircuits, a portion of which is common, a source of audio frequencycurrent for impression on said individual plate circuits, a detectoradapted to have impressed thereon said audio frequency current, theoutput of said detector being connected to the grid circuits of saidpair of vacuum tubes, and means intermediate said pair of vacuum tubesand said detector for impressing on said grids respectively in phaseopposition the detected audio currents and on the grid of one tube ofsaid pair a current substantially proportional to the variations inamplitudes of said audio frequency current and an output circuitconnected to said common portion of said plate circuits.

3. A sound reproducing circuit comprising a source of audio frequencycurrent, a bridge circuit having a pair of vacuum tubes constituting twoarms thereof, the anode cathode circuit of each tube constituting theimpedance of each arm, means for impressing said audio frequencycurrents on the other two arms of said bridge, an output impedanceconnected intermediate said tubes and said last mentioned arms, adetector circuit, means for impressing said audio frequency currents onsaid detector circuit and means connecting said detector circuit and thegrid circuits of said pair of vacuum tubes for impressing thereon saiddetected audio frequency currents in phase opposition and on the grid ofone of said tubes, at current proportional to the amplitude of saidaudio frequency currents.

4. A sound reproducing system comprising a photosensitive device, asound reproducer, means intermediate said photosensitive device and saidreproducer for expanding the level of the currents appearing in theoutput of said photosensitive device, said means including a bridgecircuit including a pair of vacuum tubes, a detector and connectionsbetween said bridge circult and detector for impressing on the grids ofsaid tubes of said bridge circuit detected currents of saidphotosensitive device in phase opposition and a current proportional tothe amplitude of the output currents of said photosensitive device.

5. The method of obtaining a control efiect proportional to and whichvaries in accordance with the amplitude of sound frequency currentscomprising detecting said currents to produce a current whose amplitudevaries in accordance with the average amplitude of said sound variationsand deriving therefrom two potentials which vary in volume andfrequency, eliminating the volume variations from one of said derivedpotentials and combining the same with said other derived potentials inphase opposition as respects audio frequency variations, and derivingsaid control effect from said combined potentials.

6. The method of expanding the volume level of sound currentstransmitted through a bridge circuit comprising detecting said soundcurrents simultaneously with their impression upon said bridge circuitand impressing on said bridge circuit the alternating frequencycomponents of said detected currents in phase opposition and a currentproportional to the average volume level of said sound currents forunbalancing said bridge.

'7. The method of reproducing sound from a sound record having soundmodulations thereon and an average print transmission varying inaccordance with the amplitude of the recorded sound, said reproductionoccurring over a bridge circuit comprising translating light varying inaccordance with said sound record into electrical currents, impressingthe alternating current components of said currents upon said bridgecircuit and impressing the alternating current components of thedetected currents upon said bridge circuit in phase opposition and alsoa detected impulse which varies with the average print transmission ofsaid record.

8. In an apparatus for reproducing sound from a sound record, means fortranslating the sound record into varying electrical currents, means fordetecting said current, a bridge circuit, means for passing thealternating components of said detected current into opposite sides ofsaid bridge circuit, means for passing the direct current component ofsaid detected current into one side of said bridge circuit, and anoutput circuit across said bridge circuit,

9. In a sound apparatus a source of audio frequency current, a soundreproducer, means intermediate said source and said reproducer forexpanding the volume level of said audio current, said means comprisinga detector adapted to have impressed thereon said audio current, a pairof vacuum tubes, the output of said detector being connected to thegrids of said vacuum tubes, a transformer between said detector and thegrid of one of said tubes, a common plate circuit for said tubes, meansfor impressing said audio current on said plate circuits, and meansconnecting said plate circuits to said reproducer.

10. A sound apparatus comprising a source of audio frequency current, adetector adapted to have impressed thereon said audio current, aresistance in the plate circuit of said detector, a pair of vacuumtubes, the grids of said vacuum tubes being connected across saidresistance, a bridge circuit, the plate circuits of said vacuum tubescomprising two arms of said bridge, means for impressing said audiocurrent on either arm of said bridge, and an output circuit across saidbridge.

11. In an apparatus for reproducing sound from a sound record, aphoto-electric device for translating the sound record into an audiofrequency current, a transmission circuit for said audio current, a pairof vacuum tubes having their cathode-anode circuits connected to saidtransmission circuit, a detector tube, means for impressing on the gridof said detector tube said audio current, a transformer having itsprimary winding in the cathode-anode circuit of said detector tube,means for impressing on the grid of one of said vacuum tubes the currentflowing in the cathode-anode circuit of said detector tube and means forimpressing on the grid of the other of said vacuum tubes the currentflowing through the secondary winding of said transformer.

12. In an apparatus for. reproducing sound from a sound record, aphoto-electric device for translating the sound record into an audiofrequency current, a transmission circuit for said audio current, adetector tube, means for impressing said audio current on the grid ofsaid tube, a transformer and resistance in the cathode-anode circuit ofsaid tube, a resistance in the secondary circuit of said transformer,and means for impressing on said transmission circuit a currentcontrolled by the drop in potential across both of said resistances.

13. An apparatus for reproducing sound from a sound record comprising aphoto-electric device for translating the sound record into an audiofrequency current, a transmission circuit for said audio current, a pairof vacuum tubes having their cathode-anode circuits connected to saidtransmission circuit, a detector tube, means for impressing said audiocurrent on the grid of said detector tube, a resistance in thecathode-anode circuit of said detector tube, and means connecting thegrids of said vacuum tubes across said resistance whereby thealternating current components impressed upon the grid of one of saidvacuum tubes will be in phase opposition with the alternating currentcomponents impressed on the grid of said other vacuum tube.

JAMES R. BALSLEY.

